Process and device for the combined production of hydrogen and carbon dioxide from a hydrocarbon mixture

Abstract

The invention relates to a process for the combined production of hydrogen and carbon dioxide from a hydrocarbon mixture, in which the residual gas of a PSA H.sub.2 (12) is separated by permeation in order to reduce the hydrocarbon content thereof and the hydrocarbon-purified gas is separated at a low temperature to produce a carbon dioxide-rich liquid (22).

Claims

1. A process for the combined production of hydrogen and carbon dioxide from a mixture of hydrocarbons comprising: (a) reforming or partially oxidizing the mixture of hydrocarbons in order to obtain a synthesis gas containing at least hydrogen, carbon monoxide, carbon dioxide, methane, steam and at least one of the following hydrocarbons: ethane, propane, ethylene, propene, benzene or methanol, (b) cooling the synthesis gas with recovery of the available heat, (c) shift reacting all or part of the cooled synthesis gas in order to oxidize most of the carbon monoxide to give carbon dioxide with corresponding production of hydrogen and of a synthesis gas enriched in H.sub.2 and CO.sub.2 and containing impurities, including at least one of the following hydrocarbons: ethane, propane, ethylene, propene, benzene or methanol, (d) cooling the synthesis gas enriched in H.sub.2 and CO.sub.2 resulting from step (c) with removal of condensed water, (e) additionally drying the cooled synthesis gas in order to obtain a dry synthesis gas, (f) separating the dry synthesis gas in a pressure swing adsorption unit, thereby obtaining a high-pressure H.sub.2 stream enriched in hydrogen and a PSA residual gas stream depleted in hydrogen containing at least carbon dioxide, hydrogen and at least one of the following hydrocarbons: ethane, propane, ethylene or propene, (g) drying the residual gas stream, (h) separating by permeation the stream of the residual gas in a first membrane system, thereby obtaining a permeate depleted in at least one of the following hydrocarbons: ethane, propane, ethylene, propene, benzene or methanol and enriched in carbon dioxide and in hydrogen, as well as a non-permeate enriched in at least one of the following hydrocarbons: ethane, propane, ethylene or propene and depleted in carbon dioxide and in hydrogen, the permeate having a carbon dioxide partial pressure of greater than 519 kPa, and (i) partially condensing and/or distilling the permeate in order to obtain a liquid stream rich in CO.sub.2 and a residual gas stream.

2. The process according to claim 1, wherein there is no step of compression of the permeate between steps (h) and (i).

3. The process according to claim 1, wherein the non-permeate from step (h) is separated in a second membrane system.

4. The process according to claim 1, wherein the non-permeate from step (h) is partially condensed.

5. The process according to claim 1, wherein at least a part of the non-permeate from step (h) or a gas derived from this non-permeate is sent to the reforming or partial oxidation step, as fuel for a burner of this step.

6. The process according to claim 1, wherein at least a part of the non-permeate from step (h) or a gas derived from this non-permeate is sent to the reforming or partial oxidation step as feed gas.

7. The process according to claim 1, wherein the first and/or a second membrane system removes at least one of the following hydrocarbons: ethane, propane, ethylene, propene, benzene or methanol, so that at least 80% of the at least one hydrocarbon feeding the membrane system is re-encountered in the non-permeate from the first and/or second membrane system.

8. The process according to claim 1, further comprising a step of compression in a compressor upstream of the first membrane system.

9. The process according to claim 8, wherein a fluid derived from the non-permeate from the first membrane system is recycled upstream of the compressor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other characteristics and advantages of the present invention will become apparent on reading the description below of nonlimiting implementational examples, descriptions made with reference to the appended figures, in which:

(2) FIG. 1 is a diagrammatic view of a process for the combined production of hydrogen and carbon dioxide as described in WO2006/054008 (state of the art).

(3) FIG. 2 is a diagrammatic view of a process for the combined production of hydrogen and carbon dioxide according to a base configuration of the invention.

(4) FIG. 3 is a diagrammatic view of a process according to the invention which differs from that of FIG. 2 in that a second membrane stage makes it possible to improve the recovery of hydrogen and of carbon dioxide.

(5) FIG. 3A shows in more detail the last part of the process of FIG. 3.

(6) FIG. 4 is a diagrammatic view of a process according to the invention which differs from that of FIG. 2 in that a second partial condensation and a second membrane stage make it possible to improve even more the recovery of hydrogen and of carbon dioxide from the gas. The case has also been represented where the fraction rich in hydrogen is purified in an independent PSA H.sub.2, so as to avoid an additional modification of the initial unit in the event of renovation.

(7) FIG. 4A shows in more detail the last part of the process of FIG. 4.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(8) FIG. 1 describes the state of the art in which a feedstock of hydrocarbons 1 mixed with steam (not represented) feeds a reformer 2 in order to generate a synthesis gas 3 containing at least carbon monoxide, hydrogen, carbon dioxide, unreacted methane and impurities. This steam reforming stage is carried out in a steam reforming furnace containing tubes filled with catalysts, the heat necessary for the reforming being supplied by combustion.

(9) The synthesis gas 3 is then cooled in 4, the cooled synthesis gas 5 subsequently being subjected, in 6, to a shift reaction during which the carbon monoxide reacts with water (represented but not referenced) in order to be—partly—converted into hydrogen and carbon dioxide. The reaction involved (CO+H.sub.2O—>CO.sub.2+H.sub.2) is called water gas reaction or shift reaction. This conversion reaction is generally carried out on synthesis gas at high temperature (HT shift) or at medium temperature (MT shift); a second shift stage 6b can be carried out downstream of the preceding one, on the partially converted synthesis gas, at lower temperature (low temperature shift)—this second conversion stage 6b is represented in broken lines; it is not compulsory. The synthesis gas obtained 7—at the outlet of stage 6 or of stage 6b when stage 6 is followed by a stage 6b—is enriched in H.sub.2 and CO.sub.2 and depleted in CO; it is cooled in 8, then the cooled gas 9 is dried in 10 (for example using a TSA type adsorption process) in order to remove the water molecules and to thus obtain a dry gas mixture 11—in view of the downstream treatment of the gas—which dry gas mixture is subsequently subjected to a separation stage in a pressure swing adsorption or PSA H.sub.2 unit 12 in order to produce a gas stream 16 of hydrogen produced and a PSA residual gas stream 14 (residual Rpsa). The stream 14 is subsequently treated in order to capture the carbon dioxide therefrom; for this, it is compressed (substage 13a), so that its pressure is between 20 and 100 bar, and it undergoes a substage 13b of purification by adsorption, so as to remove certain heavy impurities, such as benzene or methanol.

(10) Subsequently, it undergoes one or more successive stages of condensation/separation in the CPU unit 21 in order to obtain a liquid stream 22 enriched in CO.sub.2 and a gas stream 20 (residual RI) enriched in hydrogen and other non-condensable constituents, in particular in carbon monoxide and methane. The stream 20 is subsequently subjected to a stage of separation in a permeation unit 8 through a membrane in order to produce a gas stream 23 (permeate) enriched in hydrogen (Hii stream) and a gas stream 19 enriched in carbon monoxide and in methane. This gas stream 19 can, for example, be sent as fuel to the reformer 2. The drying of the synthesis gas, upstream and/or downstream of the PSA (not represented), makes it possible to remove the water which is harmful to the good progression of the downstream process.

(11) In FIG. 2 according to the invention, a feedstock of hydrocarbons 1 mixed with steam (not represented) feeds a reformer 2 in order to generate a synthesis gas 3 containing at least carbon monoxide, hydrogen, carbon dioxide, unreacted methane and impurities, including at least the following hydrocarbons: ethane, propane, ethylene or propene.

(12) This steam reforming stage is carried out in a steam reforming furnace containing tubes filled with catalysts, the heat necessary for the reforming being supplied by combustion. The synthesis gas 3 is then cooled in 4, the cooled synthesis gas 5 subsequently being subjected, in 6, to a shift reaction during which the carbon monoxide reacts with water (represented but not referenced) in order to be—partly—converted into hydrogen and carbon dioxide. The reaction involved (CO+H.sub.2O—>CO.sub.2+H.sub.2) is called water gas reaction or shift reaction. This conversion reaction is generally carried out on synthesis gas at high temperature (HT shift) or at medium temperature (MT shift); a second shift stage 6b can be carried out downstream of the preceding one, on the partially converted synthesis gas, at lower temperature (low temperature shift)—this second conversion stage 6b is represented in broken lines; it is not compulsory. The synthesis gas obtained 7—at the outlet of stage 6 or of stage 6b when stage 6 is followed by a stage 6b—is enriched in H.sub.2 and CO.sub.2 and depleted in CO; it is cooled in 8, then the cooled gas 9 is dried in 10 (for example using a TSA type adsorption process) in order to remove the water molecules and to thus obtain a dry gas mixture 11—in view of the downstream treatment of the gas which dry gas mixture is subsequently subjected to a separation stage in a pressure swing adsorption or PSA H.sub.2 unit 12 in order to produce a gas stream 16 of hydrogen produced and a PSA residual gas stream 14 (residual Rpsa). The stream 14 is subsequently treated in order to capture the carbon dioxide therefrom; for this, it is compressed in a compressor 13 so that its pressure is between 20 and 100 bar, in order to produce the gas 17. It can undergo a substage of purification by adsorption so as to remove certain heavy impurities, such as benzene or methanol. Subsequently, it is separated in a membrane system 8 in order to produce a permeate 20 enriched in carbon dioxide and in hydrogen and a non-permeate depleted in carbon dioxide and in hydrogen and containing at least 90% of the at least one hydrocarbon present in the gas 17. The permeate 20, having a CO.sub.2 partial pressure of at least 519 kPa, is not compressed and undergoes one or more successive stages of condensation/separation in the CPU unit 21 in order to obtain a liquid stream 22 enriched in CO.sub.2 and a gas stream 23 enriched in hydrogen and in other non-condensable constituents, in particular in carbon monoxide and methane. The stream 23 is returned to the adsorption unit 12 in order to separate it with the gas 11.

(13) The non-permeate 19 enriched in at least one of the following hydrocarbons: ethane, propane, ethylene or propene and depleted in carbon dioxide and in hydrogen is sent as fuel to the furnace 2.

(14) The membrane of the membrane system 17 can operate between ambient temperature and 100° C., preferably in the vicinity of 80° C., for example between 70° C. and 90° C. The membrane can be a polymer membrane capable of separating the hydrogen which can be a polyamide, polyaramid, polybenzimidazoles, mixture of polybenzimidazole and polyamides.

(15) In FIG. 3, only the differences from FIG. 2 will be described. The non-permeate 19 from the first membrane system 17 is sent to a second membrane system 24. The non-permeate 19 enriched in at least one of the following hydrocarbons: ethane, propane, ethylene or propene and depleted in carbon dioxide and in hydrogen is separated in the second membrane system 24. The permeate 25 from the second membrane system 24 enriched in CO.sub.2 with respect to the non-permeate 19 is returned to the compression stage 13. The non-permeate 26 from the second membrane system 24 is sent to the reforming 2.

(16) In FIG. 3A, the processing stages 4, 6, 8 and 10 of FIG. 3 are omitted: on the other hand, stages 12 to 24 are illustrated in more detail.

(17) After the stage of adsorption in the unit 12, which produces hydrogen 16 and a flow 14 depleted in hydrogen, the flow 14 is compressed by the compressor 13A, purified of water (case where the drying is downstream of the PSA) and/or of methanol and/or of benzene in the adsorber 13C and compressed again by the compressor 13B. The flow 17 produced by the compressor 13B is separated in order to produce a flow 20 enriched in CO.sub.2 and depleted in at least one hydrocarbon. The flow 20 is cooled by the cooler 31, the cooled flow 33 is partially condensed and separated in a phase separator 32, the gas 23 from which is returned to the adsorption 12. The liquid 34 is separated by distillation in a distillation column 35 in order to produce a gas 36 and a liquid rich in carbon dioxide 22. The non-permeate 19 from the first membrane system 8 is sent to a second membrane system 24, the permeate 25 from which is sent upstream of the compressor 13B and the non-permeate 26 from which is sent to the reformer 2.

(18) The following table shows data for the process of FIGS. 3 and 3A.

(19) TABLE-US-00001 TABLE 1 FIG. 3 Membrane 8 Membrane 24 Name 17 19 20 19 26 25 Steam 1 1 1 1 1 1 Temperature [C.] 82.00 79.22 80.93 79.22 67.42 75.22 Pressure [bar] 47.30 45.79 25.60 45.79 45.50 9.89 Molar flow [Nm3/h*] 31 426 17 346 14 080 17 346 5943 11 403 Mass Flow [kg/h] 41 173 22 560 18 614 22 560 6504 16 056 Molar Concentration CO.sub.2 0.53941269 0.47232622 0.62206223 0.47232622 0.16488324 0.63257652 Nitrogen 8.09E−03 1.28E−02 2.28E−03 1.28E−02 2.77E−02 5.02E−03 CO 0.10764726 0.16048029 4.26E−02 0.16048029 0.3140941 8.04E−02 Hydrogen 0.22444731 0.15521411 0.3097416 1.55E−01 1.59E−02 2.28E−01 Methane 0.12034078 0.19906286 2.34E−02 1.99E−01 4.77E−01 5.42E−02 Ammonia 3.21E−10 2.42E−10 4.20E−10 2.42E−10 4.07E−11 3.46E−10 Ethane 4.66E−05 8.28E−05 1.99E−06 8.28E−05 2.32E−04 5.02E−06 Ethylene 2.87E−06 4.72E−06 5.97E−07 4.72E−06 1.11E−05 1.37E−06 Propane 3.86E−06 6.97E−06 3.37E−08 6.97E−06 2.02E−05 8.64E−08 Propene 2.16E−06 3.88E−06 3.60E−08 3.88E−06 1.12E−05 9.19E−08 Methanol 1.10E−05 1.93E−05 7.83E−07 1.93E−05 5.27E−05 1.94E−06 Benzene 9.57E−09 1.73E−08 3.29E−11 1.73E−08 5.04E−08 8.44E−11 Products Residues(%) Permeates(%) Residues (%) Permeates(%) CO.sub.2 48.33 51.67 11.96 88.04 Nitrogen 87.37 12.63 74.21 25.79 CO 82.29 17.71 67.06 32.94 Hydrogen 38.17 61.83 3.52 96.48 Methane 91.30 8.70 82.11 17.89 Ammonia 41.50 58.50 5.78 94.22 Ethane 98.08 1.92 96.02 3.98 Ethylene 90.68 9.32 80.85 19.15 Propane 99.61 0.39 99.19 0.81 Propene 99.25 0.75 98.44 1.56 Methanol 96.82 3.18 93.39 6.61 Benzene 99.85 0.15 99.68 0.32

(20) Thus, it is found that more than 90% of the methane, ethylene and methanol in the flow 17 and more than 98% of the ethane, propane, propene and benzene in the flow 17 is re-encountered in the non-permeate 19 from the first membrane system 8.

(21) The second membrane system 24 is used to remove more than 98% of the ethane, propane, propene and benzene in the flow 17, which is re-encountered in the non-permeate 26.

(22) In FIG. 4, the non-permeate 19 undergoes condensation 15 in order to recycle a condensate enriched in CO.sub.215B towards the compression 13 and the non-condensables 15A pass into a second membrane 24 in order to separate a fraction rich in hydrogen 26, recycled upstream of the PSA, and a fraction 25 rich in carbon monoxide and methane, which goes to the burners of the reformer 2.

(23) In FIG. 4A, the processing stages 4, 6, 8 and 10 of FIG. 4 are omitted: on the other hand, stages 12 to 24 are illustrated in more detail.

(24) After the stage of adsorption in the unit 12, which produces hydrogen 16 and a flow 14 depleted in hydrogen, the flow 14 is compressed by the compressor 13A, purified of water (case where the drying is downstream of the PSA) and/or of methanol and/or of benzene in the adsorber 13C and compressed again by the compressor 13B. The flow 17 produced by the compressor 13B is separated in order to produce a flow 20 enriched in CO.sub.2 and depleted in at least one hydrocarbon. The flow 20 is cooled by the cooler 31, the cooled flow 33 is partially condensed and separated in a phase separator 32, the gas 23 from which is returned to the adsorption 12. The liquid 34 is separated by distillation in a distillation column 35 in order to produce a gas 36 and a liquid rich in carbon dioxide 22. This liquid can contain 99.8% of carbon dioxide at least, being of food grade.

(25) The non-permeate 19 from the first membrane system 8 is sent to a heat exchanger 37 in order to be partially condensed and is separated in a phase separator 15. The gas formed 15A is reheated in the exchanger 37 and sent to a second membrane system 24, the permeate 25 of which is sent upstream of the PSA 12 and the non-permeate 26 of which is sent to the reformer 2.

(26) The liquid 15B from the phase separator 15 is expanded and then vaporized in the exchanger 37 in order to be sent upstream of the compressor 13B.

(27) It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.